1. Field of the Invention
The invention relates to a method and apparatus for orienting a perforating device in a subterranean well. More particularly, it relates to an improvement for orienting a perforating device during completion so as to take advantage of directional reservoir characteristics.
2. Description of the Prior Art
Methods are known in the art for orienting perforating guns on downhole oil tools. Generally, such methods have sought to orient a perforating gun in a slanted or offset borehole to prevent clogging of the wellbore from perforations shot vertically upward through the casing. Two such methods are disclosed in U.S. Pat. No. 4,830,120 ("'120 patent"), issued to Stout and U.S. Pat. No. 4,637,478 ("'478 patent"), issued to George.
The '120 patent describes a method and apparatus for firing perforating charges of a gun when they have been oriented in the desired direction. The gun can be rotated by various techniques. If the gun is positioned in the well under a set packer, an explosive charge in the upper portion of the gun firing head and above a swivel may force a piston down a spiral path. The perforating charges and interconnected lower portion of the firing head are rotationally responsive to rotation of the piston, and are thus forced to rotate. A mercury switch responsive to rotation of the perforating charges and positioned below the swivel completes the circuit and the perforating charges fire in their pre-selected direction based on their rotational position relative to the mercury switch. If the well is being completed without the use of a packer it is possible to trip the initiating switch and, if the gun has not fired, rotate the tubing from the surface until the mercury switch is in its downward position, thus completing the circuit and firing the gun.
The '478 patent describes a tubing conveyed perforating gun assembly for connection to a tubing string which enables the gun to be run downhole into a slanted borehole and detonated in order to perforate the wall of the borehole in a predominately downward direction. The charges are directed predominantly downward by virtue of a charge carrier whose center of gravity is displaced from the axis of a spaced journal means in the direction of said firing pattern to cause said shape charges to gravitate into a position which orients the charges predominantly downward so that the penetration, when the charges are dedicated, occurs in a downward direction.
U.S. Pat. No. 4,523,649 ("'649 patent"), also issued to Stout, discloses a further method and apparatus for rotational alignment of tubing conveyed perforating guns. The disclosed apparatus contains a swivel unit connecting a perforating gun assembly to the end of a tubular conduit, said swivel unit comprising an angular thrust bearing thereby permitting rotation of the perforating assembly relative to the conduit. An axially extending narrow rib on the exterior of said perforating gun assembly is aligned with and overlays a portion of one row of charge containers. The containers overlayed by said rib are blank, which shifts the center of gravity to cause said perforating gun assembly to gravitationally rotate in any non-vertical section of the well casing to position said rib in engagement with the upper portion of the non-vertical casing section. The rib is angularly positioned relative to all the explosive charge containers so that no explosive charge is directed vertically upward.
The above-described methods, while useful in many situations, do not offer the degree of control needed to take advantage of directional reservoir characteristics, particularly where the wellbore is substantially vertical.
A method such as that described in the '120 patent is even disadvantageous. The '120 patent, for example, provides for the orientation procedure to occur as a part of the procedure for initiating the explosive charges. As a result, operators on the surface are unable to confirm the orientation of the charges at a particular azimuth prior to the charges being fired.
Methods such as those described in the '478 patent and the '649 patent rely upon gravity to orient the perforating gun charges and, consequently, require a non-vertical section of the well casing to do so. Also, charges are oriented primarily downward rather than in a geographical direction as required to exploit a reservoir's geographic characteristics. Clearly, means are needed to permit drilling crews attempting to develop a formation in a geographically advantageous manner to orient well completions azimuthally, with respect to magnetic north.